Chlorine dioxide has long been recognized as an oxidizing treatment having great utility in a variety of applications. Chlorine dioxide is used in the paper industry as a bleaching agent for paper pulp, in the water and waste treatment industry as a biocide for bacteria, algae and various water-borne microorganisms, in the fat rendering and tallow industry as both a biocide and bleaching agent, and generally as an oxidant useful in destroying certain organic materials, such as phenols. In whatever application chlorine dioxide is used, the need for treatment is likely to fluctuate depending on variations in the amount of material being treated, the degree of pollution or contamination of the material, the degree of bleaching required, and so forth.
The process of treating water and other materials with chlorine dioxide consists generally of introducing and mixing a quantity of chlorine dioxide with the material. wherein the quantity of chlorine dioxide is sufficient to completely and effectively treat the material. In some applications, chlorine dioxide is sufficient to completely and effectively treat the material. In some applications, chlorine dioxide is continuously generated in quantities large enough to meet the treatment requirements of peak demand. There are problems associated with operating a chlorine dioxide generator in this manner. During periods of low demand. Operating in an overtreatment mode is uneconomical, inefficient, and may even result in excessive wear and tear on mechanical and electronic equipment and those materials exposed to the high concentrations of ClO.sub.2 and reactants. During periods of extremely high demand, undertreatment conditions may result, leaving materials incompletely sanitized or bleached.
To overcome the problems inherent in the fixed rate generation of chlorine dioxide, chlorine dioxide generator operators can manually adjust the generation rate in response to variations noted from periodic measurements of the need for treatment. The measurements may be based on, for example, the amount of material requiring treatment, the amount of pollutants needing treatment, and the oxidation-reduction potential of the water. However, unless the measurements of the need for treatment and the accompanying adjustments have a high frequency, or coincide fortuitously with rapid increases and decreases in the need for treatment, over and undertreatment conditions may occur.
Automated systems have been developed for supplying chlorine dioxide on an as needed basis to treat water. One such system known to the inventors comprises a chlorine dioxide generator which operates on a constant rate basis, sending an aqueous stream of ClO.sub.2 produced by the generator to a holding tank. The volume of ClO.sub.2 solution in the tank released to the water treatment site is based on a signal received from a treatment need detecting device, such as a flow monitoring device in the water treatment sluice. In the event the holding tank becomes filled with chlorine dioxide solution, the generator is shut down until more ClO.sub.2 is needed.
Because chlorine dioxide is released from water solution on standing, a ClO.sub.2 solution held in the tank will lose strength and hence effectiveness, thus making complete treatment of materials with that solution uncertain. The chlorine dioxide which has left the solution may become trapped in the free space of the holding tank if not properly vented. Should the concentration of ClO.sub.2 in the air above the tank reach a critical level, a spontaneous explosion of the chlorine dioxide may occur.
An automated ClO.sub.2 generating system was developed by Rio Linda (Fischer & Porter Co., Warminster, Pa.) and installed at a drinking water treatment facility in Shreveport, La. The Shreveport system is described in an article entitled "Automated Approach for ClO.sub.2 Disinfection" found in the October, 1988 issue of Water Engineering Magazine, pp. 35-38. It appears that the generator equipment of the Rio Linda Shreveport system is of the vacuum eductor type which uses the chlorite-chlorine route to produce ClO.sub.2.
The control portion of the Rio Linda Shreveport system operates by receiving signals from a water flow monitoring device and simultaneously adjusting mechanical valves associated with the supply sources of both the sodium chlorite and chlorine gas reactants feeding the generator. It is well known that it is difficult to mechanically control gas flowing through a valve to the degree of accuracy needed to achieve the desired 2:1 ratio of reactants in order to operate the system at the most desired levels of efficiency and economy. Furthermore, the control system associated with the supply valves does not take into account the flow differences caused by the level of reactants in their respective tanks, the pressure at which the materials are delivered, or the relative concentrations of the reactants. The result is that the reactants are not used economically.
It will be well understood by practitioners of the art of oxidation treatment that the equipment and processes used in the treatment of water with ClO.sub.2 may be used in the paper and pulp industry, the fat rendering industry and others. It is believed that little or no modification is necessary to adapt a method and apparatus for use in applications other than water treatment.